This invention relates to a digital multi-level multi-phase modulation communication system.
Conventionally, digital multi-level multi-phase modulation communication systems use coherent detection for phase demodulation at the receiving end because of its advantage in respect of the carrier to noise ratio (C/N ratio) required. However, due to the phase ambiguity in phase-locking that occurs during the reproduction of the reference carrier, the coherent detection system sometimes produces a demodulated pulse pattern differing from the pulse pattern supplied. To prevent this phenomenon, the prior art has used the differential encoder/decoder circuitry. For instance, according to the 2.sup.m -phase PSK modulation system (m is 1, 2, 3 . . . ), the pulse pattern fed is subjected to 2.sup.m -ary differential encoding, and the pulse train demodulated at the receiving end is subjected to 2.sup.m -ary differential decoding to thereby reproduce the original signal.
The 16 QAM modulation communication system is most popular as the 2.sup.m -phase PSK modulation system. Since the system has 12 phases, it requires theoretically duodenary differential encoding, but it actually needs only quaternary differential encoding because, in order to provide 4-level modulated signals, the phase ambiguity in phase-locking that occurs to the reference carrier reproduced must be limited to 4 phases at .pi./2 radians. This feature is embodied in one conventional type of 16 QAM communication system that uses two quaternary differential encoder circuits at the transmitting end and uses two quaternary differential decoder circuits at the receiving end to demodulate the received signal into the original signal. The system uses a very simple differential coding (encoding/decoding) operation, but it has a bit error rate twice as high as the other systems because all four trains of the baseband digital input signal undergo the quaternary differential encoding/decoding. For details of the above described 16 QAM communication system, reference is made to:
(1) I. Horikawa et al., "Characteristics of a High Capacity 16 QAM Digital Radio System on a Multi-path Fading Channel," ICC '79 Conference Record, Volume 3 or 4, pp. 48.4.1-48.4.6, June 10-14, 1979;
(2) Japanese patent application Disclosure No. 109811/77 (disclosing the technique reported in reference (1))